To get in line with the global trend of environmental protection and eco-friendly regulations, electronic product manufacturers nowadays are developing and manufacturing halogen-free electronic products. Advanced countries and electronic manufacturing giants set forth schedules of launching mass production of halogen-free electronic products. As a result of the promulgation of the Restriction of Hazardous Substances (RoHS) by the European Union, hazardous substances, such as lead, cadmium, mercury, hexavalent chromium, poly-brominated biphenyl (PBB), and poly-brominated diphenyl ether (PBDE), are strictly prohibited from being used in manufacturing electronic products or their parts and components. A printed circuit board (PCB) is an indispensable and fundamental basis of the semiconductor industry and electronic industry; hence, printed circuit boards bore the brunt of international halogen-free regulations when international organizations set forth strict requirements of the halogen content of printed circuit boards. For example, the International Electrotechnical Commission (IEC) 61249-2-21 requires that bromine content and chloride content shall be less than 900 ppm and the total halogen content shall be less than 1500 ppm. The Japan Electronics Packaging and Circuits Association (JPCA) requires that both bromide content and chloride content shall be less than 900 ppm. To enforce its green policies, Greenpeace calls on manufacturers worldwide to get rid of polyvinyl chloride (PVC) and brominated flame retardants (BFRs) from their electronic products in order to conform with the lead-free and halogen-free requirements of green electronics. Hence, the industrial sector nowadays is interested in rendering related materials halogen-free and sees this technique as one of its key research topics.
Electronic products nowadays have the trend toward compactness and high-frequency transmission; hence, circuit boards nowadays typically feature a high-density layout and increasingly strict material requirements. To mount high-frequency electronic components on a circuit board, it is necessary that the substrate of the circuit board is made of a material of a low dielectric constant (Dk) and dielectric dissipation factor (Df) in order to maintain the transmission speed and the integrity of a signal transmitted. To allow the electronic components to function well at a high temperature and a high-humidity environment, it is necessary for the circuit board to be heat resistant, fire resistant, and of low hygroscopicity. Epoxy resin is adhesive, heat resistant, and malleable and thus is widely applicable to encapsulants and copper clad laminates (CCL) of electronic components and machinery. From the perspective of fire prevention, epoxy resin is incapable of flame retardation, and thus epoxy resin has to acquire flame retardation capability by including a flame retardant therein. For example, a halogen, such as bromine, is included in epoxy resin to bring about flame retardation capability thereof.
Recent years see electronic products becoming lighter and smaller and circuits becoming finer, thereby disadvantaging heavy halogen compounds. When exposed to a high temperature for a long period of time, a halogen compound is likely to decompose and thereby erode a fine circuit. Furthermore, combustion of discarded used electronic parts and components produces hazardous compounds, such as halogen compounds, which are environmentally unfriendly. To find an alternative to the aforesaid halogen compound-based flame retardant, researchers attempt to use a phosphorus compound as a flame retardant, for example, adding phosphate ester (U.S. Pat. No. 6,440,567) or red phosphorus (EP patent No. 0763566) to an epoxy resin composition. However, phosphate ester undergoes hydrolysis readily to produce an acid, thereby compromising its tolerance to migration. Although red phosphorus is good at flame retardation, it falls into the category of hazardous compounds under the firefighting law, because it produces a trace of a flammable, toxic gas known as phosphine in a warm humid environment.
A conventional circuit board manufacturing method, such as a conventional method of manufacturing a copper-clad substrate (also known as copper clad laminate, CCL), involves heating and combining a reinforcement material (such as a glass fabric) and a thermosetting resin composition made of an epoxy resin and a curing agent to form a prepreg, and then laminating the prepreg and the upper and lower copper foils together at a high temperature and a high pressure. The prior art usually teaches using a thermosetting resin composed of an epoxy resin and a hydroxyl-containing phenol novolac resin curing agent. Due to the combination of the phenol novolac resin and the epoxy resin, epoxide ring-opening reactions end up with another hydroxyl which not only increases the dielectric constant (Dk) and the dielectric dissipation factor inherently, but also reacts with water readily and thereby renders the thermosetting resin more hygroscopic.
U.S. Pat. No. 7,255,925 discloses a thermosetting resin composition composed of cyanate ester resin, dicyclopentadiene (DCPD) epoxy resin, silica, and a thermoplastic resin. The thermosetting resin composition is characterized by a low dielectric constant (Dk) and a low dielectric dissipation factor. However, a method for manufacturing the thermosetting resin composition of U.S. Pat. No. 7,255,925 requires the use of a halogen-containing (such as bromine-containing) flame retardant, such as tetrabromocyclohexane, hexabromocyclodecane, or 2,4,6-tris(tribromophenoxy)-1,3,5-triazine. However, the bromine-containing flame retardant causes environmental pollution readily during the thermosetting resin composition manufacturing process, the using processing of thermosetting resin composition, and even after the thermosetting resin composition has been discarded or recycled. To ensure a low dielectric dissipation factor, low hygroscopicity, high cross-linking density, high glass transition temperature, high connectivity, appropriate thermal expansion, heat resistance, and fire resistance of copper clad laminates, it is necessary to select an epoxy resin, a curing agent, and a reinforcement material carefully.
The major considerations given to electrical properties include the dielectric constant (Dk) and the dielectric dissipation factor. In general, the signal transmission speed of a substrate is inversely proportional to the square root of the dielectric constant (Dk) of the material from which the substrate is made, and thus the minimization of the dielectric constant (Dk) of the substrate material is usually advantageously important. The lower the dielectric dissipation factor is, the lesser the signal transmission attenuation is; hence, a material of a low dielectric dissipation factor provides satisfactory transmission quality.
Accordingly, it is important for printed circuit board material suppliers to develop materials of a low dielectric constant (Dk) and a low dielectric dissipation factor, and apply the materials to high-frequency printed circuit board manufacturing.